Pioneering Plug-In Hybrids

Pupil transporters across the country are taking delivery of the first wave of plug-in hybrid school buses. Officials involved with the project expect significant reductions in fuel use, emissions and even brake wear.

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Emily Mulrine is excited to have the name she chose, "Limpio," emblazoned in green letters on the side of a school bus.

But, more importantly, she's excited about what's inside the bus: a hybrid-electric powertrain.

She's also excited about what will be coming out of the bus: an estimated 90-percent less emissions.

And, of course, she's excited about what will be going into the bus: far less diesel, due to a projected 70- to 100-percent increase in fuel economy.

Mulrine, a sixth-grader in the Manatee County (Fla.) School District, was one of the winners of a contest to name the district’s two new plug-in hybrid school buses — which are among the first school buses of their kind in the country.

Mulrine's moniker, "Limpio," means "clean" in Spanish. Her co-winner, high-schooler William Ung, submitted the name "Wouk" in honor of Victor Wouk, a pioneer in developing hybrid technology.

"It's exciting that the name I chose is going to be on the hybrid bus, and it's great that these buses are going to help the environment," Mulrine says.

In all, 19 of these new buses have been awarded to school transportation providers across the nation by Advanced Energy, a non-profit corporation that initiated the groundbreaking Plug-In Hybrid Electric School Bus Project.

How they work The plug-in hybrid Type C school buses, which were built by bus manufacturer IC Corporation and hybrid drivetrain maker Enova Systems, have a charge-depleting system that draws on energy stored in the batteries during the driving cycle to optimize fuel economy.

With this type of system, more energy can be taken from the batteries than in a standard, or charge-sustaining, hybrid —but for a limited distance (about 44 miles). Once the batteries have been "pulled down," the system will maintain itself in sustaining mode. Then the batteries are plugged in for recharging.

The hybrid buses also use regenerative braking to capture some of the energy created when the driver applies the brakes, helping to charge the battery. Another advantage of regenerative braking is that it leads to longer brake life due to decreased wear. With the frequent stops of typical school bus routes, the vehicles stand to benefit significantly from this technology.

"If we assume 7 miles per gallon, which is heavily dependent on the route, buses using a charge-depleting system could see up to 12 to 14 miles per gallon during the deep discharge phase of operation," Ray says. "Again, this is dependent on a route with multiple stops per mile."

The buses couple an International VT365 V8 diesel engine with the hybrid-electric powertrain, which comprises a transmission, batteries and an electric motor. The battery pack consists of 28 individual batteries. Ray says that the pack should last 6 to 8 years in a charge-depleting system using lithium-ion batteries.

The system is based on parallel architecture, allowing it to efficiently use both diesel and electric power.

Ewan Pritchard, hybrid program manager for Advanced Energy, says that most of a vehicle's emissions are created when it is used at peak power — i.e., when it accelerates. With these hybrids, the electric motor does most of the acceleration, which reduces emissions peaks and saves fuel.

Also, biodiesel can be used in these hybrids, further reducing emissions and use of petroleum diesel. Manatee County is using a B20 biodiesel blend in its two hybrids.

Hybrid arrivals Manatee County received its two hybrid school buses in March. Don Ross, the district's supervisor of vehicle maintenance, says that the vehicles' performance has been promising so far.

"There has been no down-time whatsoever on either bus," Ross says.

The main difference in operating the buses is that drivers engage or disengage the hybrid system as they feel necessary by flipping a switch on the dash. Also, drivers notice stronger acceleration off of idle, and they feel regenerative braking occurring when they step off of the accelerator.

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